(1) Field of the Invention
The invention relates to an LED driving circuit, and more particularly relates to an LED driving circuit with protection function.
(2) Description of the Prior Art
FIG. 1 is a schematic diagram of a conventional LED driving circuit disclosed in U.S. patent publication No. 20120049741. The LED driving circuit comprises a power supply unit 302. The power supply unit 302 has a dimming output terminal DIM, a feedback terminal FB and an output terminal OUT. The feedback terminal FB receives a lowest feedback signal. The power supply unit 302 generates a driving voltage at the output terminal OUT based on an input voltage and the lowest feedback signal for driving a plural LED string. A current balance circuit 301 has an input terminal and plural output terminals. The input terminal is configured to receive a reference voltage, and the output terminals are coupled to a corresponding negative terminal of the LED strings for regulating currents of the LED strings. A feedback selector 303 has plural input terminals and an output terminal. The input terminals of the feedback selector 303 are respectively coupled to the negative terminals of the LED strings for receiving feedback signals indicative of the voltages drop of the LED strings. The feedback selector 303 generates the lowest feedback signal at the output terminal based on the received feedback signals.
The power supply unit 302 regulates the driving voltage based on the lowest feedback signal, and thus the driving voltage is at the lowest voltage value that is enough to drive the plural LED string. The current balance circuit 301 has plural current sources CS, and each current source CS is coupled to the negative terminal of a corresponding LED string for regulating the current of the corresponding LED string. A first terminal of a current setting resistance Rset is coupled to all of the current sources CS, and a second terminal thereof is coupled to the ground. The currents, provided by the current sources CS, can be adjusted by adjusting the resistance value of the current setting resistance Rset. The dimming output terminal DIM of the power supply unit 302 is adapted to provide a dimming signal to each of the current sources CS.
A protection circuit 304 is coupled between the negative terminal of the LED string and the current balance circuit 301 for protecting the current balance circuit 301 from overvoltage. The protection circuit 304 has plural transistors T each of which has a first terminal, a second terminal and a control terminal. The control terminals are coupled to a preset voltage Vg, the first terminals are respectively coupled to the negative terminals of the LED strings, and the second terminals are coupled to the current balance circuit 301. The withstand voltages of the transistors T are higher than the driving voltage generated by the power supply unit 302. When the LED string is short circuited, the protection circuit 304 endures most of the driving voltage, and protects the current balance circuit 301 from being damaged by directly coupling to the driving voltage.
However, because the current balance circuit 301 still operates normally, the transistors T of the protection circuit 304 not only endure a voltage which is almost equal to the driving voltage, but also flowed with a preset current for driving the LED strings, thus causing the transistors T to have excessive power consumption resulting in over-temperature damage. The damaged transistors T are not able to perform the original protection function, and the current balance circuit 301 is also damaged, thus failing to achieve the expected protection function in practice.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. In order to make the features and the advantages of the invention comprehensible, exemplary embodiments accompanied with figures are described in detail below.